This invention relates generally to electrical distribution equipment, and more specifically to a lug adapter assembly having current carrying clips (stabs) for attaching cable conductors to electric busway systems.
The use of switchgears in electrical distribution systems is well known. The switchgear houses a plurality of draw out units, with each draw out unit housing one or more switching devices, such as motor controllers and circuit breakers, which are releasably interconnected to one or more busbars. Periodically, the draw out units are removed from the switchgear to allow for maintenance of the equipment. The switching devices within the draw out units make electrical contact with the busbars through a plurality of clip connectors which extend from the draw out unit. Each clip connector is electrically connected to an electrical conductor for connection to the switching devices of the switchgear.
Clip connectors of the prior art are generally mounted within an insulative housing that is affixed to a frame of the draw out unit. Each clip connector is typically constructed of an electrically conductive material formed into a "U" shape. The two free ends of the "U"-shaped clip form contact arms, which extend within the clip housing and are aligned for electrical connection with a specific busbar. As the draw out unit is inserted into the switchgear, the busbar slides between the two contact arms, and the contact arms frictionally engage the busbar. Typically, only one clip connector attaches to each bus bar.
The design of the contact clip requires each contact arm to have a surface area in contact with the bus bar to reduce resistance to current flow and thereby prevent inefficient thermal losses. In the U-shaped contact clips found in the prior art, a contact surface is formed along the entire width of each of the two contact arms. Ideally, these contact surfaces remain parallel to the bus bar when the bus bar is engaged between the two contact arms, thus allowing the full width of the contact arm to be in contact with the bus bar. However, inaccuracies during manufacturing and mishandling during installation can cause deformities the contact arms and their contact surfaces. Such deformities allow only a small portion of the contact arm to actually contact the bus bar, resulting in an increase in resistance to current flow.
The design of the contact clip also requires that the contact arms exert a minimum compressive force (force per unit of area in contact with the bus bar) onto the busbar. This force is required to overcome the electromagnetic force generated by current flow in the contact aims, which tends to part the arms from the bus bar. Problematically, the required minimum compressive force is proportional to the amount of force required to insert the contact clips onto the busbar. In other words, clips having a high compressive force are difficult to slide onto the busbar. As a result, it is necessary to use a racking mechanism in order to develop sufficient insertion force to install the draw out unit into the switchgear. The need for a racking mechanism increases the equipment cost for switchgear installation and maintenance. In addition, the need for a racking mechanism necessitates that clearance space be provided for the racking mechanism within the switchgear. The additional clearance space increases the size and cost of the switchgear itself.